Embodiments of the present invention relate to an apparatuses and methods of generating a plurality of channels of radio frequency signals that are precisely synchronized with each other.
In the past, a signal generator apparatus has been used to generate an RF (radio frequency) signal as a test signal in order to measure an operational property of a receiver used in a RF communication system. In this signal generator apparatus, transmission data is stored in a data storage device, this data being used to generate the RF signal. The transmission data stored in the data storage device is digitally modulated and a frequency of the modulated data is upconverted for outputting as the RF signal.
FIG. 1 (Prior Art) shows a functional block diagram of a conventional signal generator apparatus 70. A data storage device 71 stores the transmission data to be used for generating the RF signal. When the signal generator apparatus 70 supplies the RF signal, the transmission data Di and Dq are read out from the data storage device 71, the transmission data Di is applied to a D/A converter 72 and the transmission data Dq is applied to a D/A converter 73.
The D/A converter 72 converts the transmission data Di into an analog baseband signal I and applies it to an IQ modulator 75. The D/A converter 73 converts the transmission data Dq into an analog baseband signal Q and applies it to the IQ modulator 75. A carrier wave signal generator 74 provides the IQ modulator 75 with a carrier wave signal Sm.
The IQ modulator 75 multiplies the carrier wave signal Sm and the baseband signal I to produce a modulated signal. In addition, the IQ modulator 75 shifts the phase of the carrier wave signal Sm by 90 degrees and multiplies the phase shifted carrier wave signal and the baseband signal Q to produce another modulated signal. These two modulated signals are added to each other to produce a modulated output signal Sa that is applied to a mixer 77.
A local oscillator 76 generates a local oscillation signal Sloa having a frequency higher than that of an output signal RFout from the signal generator apparatus 70. This local oscillation signal Sloa is applied to the mixer 77. The mixer 77 multiplies the modulated signal Sa and the local oscillation signal Sloa in order to upconvert the modulated signal Sa. The upconverted SRFa is applied to a bandpass filter (BPF) 78.
The bandpass filter 78 deletes an image component from the RF signal SRFa and applies its output to a mixer 80.
A local oscillator 79 oscillates a local oscillation signal Slob and applies this signal to the mixer 80. The local oscillator 79 can vary a frequency of the local oscillation signal Slob so that the frequency of the output signal RFout from the signal generator apparatus 70 can be varied.
The mixer 80 multiplies the RF signal SRFa and the local oscillation signal Slob to downconvert the frequency of the RF signal SRFa to a desired frequency. The signal SRFb of the desired frequency is applied from the mixer 80 through a power amplifier 81 to a lowpass filter (LPF) 82.
The lowpass filter 82 deletes an image component from the signal SRFb and applies its output to an attenuator 83. The attenuator 83 adjusts a level of the signal having no image to a desired signal level for producing the output signal RFout.
In the signal generator apparatus that generates the RF output signal by modulating the baseband signals I and Q and converting their frequencies, an offset phase loop is used for an envelop modulation of its output in order to generate a variable envelope wideband RF signal having a low noise as discussed in Japanese Unexamined Patent Publication No. 2001-45079. U.S. Pat. No. 4,843,351 discloses a signal generator apparatus having a high precision IQ (vector) modulation circuit.
The radio communication system uses the MIMO (Multiple Input Multiple Output) technology for increasing transmission speed. In the MIMO technology, a plurality of antennas transmits different data simultaneously and a receiver side restores the original signals from the interfered signals. According to the MIMO technology, a combination of antennas of a transmitter side and a receiver side is regarded as a virtual communication path. A transfer characteristic of each communication path is preliminarily evaluated, and the evaluated transfer characteristic is referred to when restoring the original signals from the interfered signals.
When different data are transmitted from multiple antennas simultaneously, the signal generator apparatus as shown in FIG. 1 is used at each antenna. For example, if two antennas transmit different data simultaneously, two signal generator apparatuses are required wherein a desired frequency output signal generated by one signal generator apparatus is transmitted via one antenna and another desired frequency output signal generated by the other one signal generator apparatus is transmitted via the other one antenna.
In a case that multiple signal generator apparatuses are used, if operation timing of the D/A converters 72, 73, the IQ modulators 75, etc. in one signal generator apparatus does not meet with that of other signal generator apparatus, a skew problem may occur and there is a possibility that the original signal cannot be restored. In order to synchronize the operations of the multiple signal generator apparatuses with each other, it is required to synchronize the clock signal for the D/A converters 72, 73, the local oscillation signals, etc. of one signal generator apparatus with those of other signal generator apparatus. For this, it is required that, for example, a sync signal is applied from one signal generator apparatus to the other signal generator apparatus so as to synchronize the clock signal, the local oscillation signal, etc. with the sync signal. Another approach is that a frequency synthesizer applies the sync signal to all the signal generator apparatuses so as to synchronize the clock signal, the local oscillation signal, etc. with the sync signal. Therefore, the radio communication system becomes complex in construction. Moreover, it is expensive to construct such a radio communication system.
Therefore, what is desired is a signal generator apparatus that generates a plurality of RF signals simply and inexpensively, the RF signals being synchronized with each other highly precisely.